In the interests of environment protection, the use of bleaching agents such as oxygen (O), a per-compound (P) such as hydrogen peroxide and ozone (Z) has recently been suggested for the bleaching of, for instance, sulphate pulp. This has led to the introduction, including on a commercial scale, i.e. full-scale, of the use of this type of bleaching agent, including those referred to above, and also in the sequence indicated above. By avoiding the use of bleaching agents containing chlorine, which finally give rise to corrosive chloride, it has proved increasingly possible to close bleach plants. The expression closing is used to denote that the (washing) liquids are treated to an increasing extent within the bleach plant. In traditional open bleaching plants, the washing liquids (waste liquors) remaining after the respective bleaching stage, including after the extraction (E) stage, are allowed to flow directly out to the recipient or, where appropriate, to an external purification measure.
It has emerged from the use of the oxidative bleaching agents exemplified above, and in particular from the use of a per-compound, that the content of metals in the pulp and/or the presence of metals in general often leads to problems. The metals that cause most problems are the transition metals, of which manganese is the most problematical due to the presence of manganese in such large amounts. Manganese, for example, occurs naturally in the raw material, i.e. in the lignocellulose material, for example in the form of wood. The process water that is used also contains manganese as a general rule, and manganese can also originate from the apparatus used in the pulp production chain. In an attempt to deal with this problem, a complexing agent stage (Q) has been introduced into the pulp treatment chain, preferably directly ahead of the peroxide bleaching stage. The addition of compexers such as EDTA, DTPA and NTA, and others at a suitable pH value, ensures that possibly free manganese ions are collected and, in particular, the manganese is converted from a fixed form in the cellulose pulp to a water soluble complexed form. Manganese complexes of the type Mn(EDTA).sup.2- or Mn(DTPA).sup.3- occur in this case. It is important, after this treatment stage, for the pulp to be washed extremely thorougly so that no significant quantities of manganese complexes and possibly free complexing agents accompany the cellulose pulp into the peroxide bleaching stage. The spent liquor generated at this point, i.e. the washing liquid from the complexing agent stage, has attracted the particular attention of experts.
The starting material for the manufacture of cellulose pulp, for example wood, as a general rule contains not only harmful metals, such as the transition metals referred to above, but also individual metals, such as mangnesium, which have a positive effect in the course of the bleaching of cellulose pulp with non chlorine-containing, oxidative bleaching agents such as hydrogen peroxide. When processing cellulose pulp to the point at which it is bleached with hydrogen peroxide, for example, every effort is accordingly made to retain the largest possible quantity of the original magnesium in the cellulose pulp. If the starting material in itself has a very low magnesium content, and/or if the magnesium is released from the cellulose pulp during one or more process stages, it is possible to add magnesium to the cellulose pulp, for example in the form of magnesium sulphate.
If, in the course of bleaching cellulose pulp, exclusive use is made of non chlorine-containing, oxidative bleaching agents, and particularly if these bleaching agents are not used in an optimal fashion, there is a risk of the strength characteristics of the bleached (including fully bleached) cellulose pulp becoming such that the cellulose pulp in question is not suitable as a starting material for special types of paper, i.e. types of paper for which the strength is of critical significance. It is possible in such cases, in place of a comparatively aggressive bleaching agent such as ozone, to use the chlorine-containing bleaching agent that is least environmentally unfriendly, i.e. chlorine dioxide (D). A major advantage of chlorine dioxide is that, at the same time as it produces a significant increase in the brightness of the cellulose pulp and also has a delignifying effect, it also leaves the cellulose pulp largely unaffected with regard to its strength, for example measured in the form of its intrinsic viscosity. The intrinsic viscosity of the cellulose pulp (measured according to standard method SCAN-C15:62) is an indirect indicator of the strength of the cellulose pulp and, to some extent also, of the strength of the paper produced from the cellulose pulp in question.